1. Field of the Invention
The present invention relates to a system for sampling binary value images of white and black and efficiently encoding and decoding them.
2. Description of the Prior Art
In an apparatus for sampling and reproducing an image (or a picture) on the basis of picture cells, it is known that the quality of a reproduced image depends on the following characteristics. (T. S. Huang, "PCM Picture Transmission", IEEE Spectrum, 12, 57-63 (1965)). That is, for improving the quality of an image having a lot of high space frequency components such as a document, it is necessary to increase the number of picture cells per unit area. However, in order to increase the number (or density) of picture cells, the sampling density of an image sensor must be increased and concurrently the transmission rate of image information and the density of picture cells in a reproducing apparatus must be increased. As a result, the cost of an image input and output apparatus having high image quality must inevitably be increased.
Generally speaking, a document image consists of only white and black portions and it is well known that there is a strong correlation between its image portions. The modified Huffman method adopted by CCITT as an image transmission encoding method makes use of this correlation.
It has been proposed by Nishikawa et al to reproduce white and black picture cells of high density from coarse sampled picture cells through the density information of their surrounding picture cells by utilizing the above mentioned correlation. (A Method For Reproducing An Image By A Solid Image Sensor, 1977, DENSHITSUSHINGAKUKAI General meeting 1011). This method divides a picture cell into four small picture cells by utilizing its surrounding eight picture cells to thereby obtain an image of high quality.
The object of the present invention is to provide a more efficient technique for a facsimile apparatus of high quality by utilizing the characteristics of a document image. That is, the conventional encoding method adopts a procedure of placing a plurality of white and black picture information in picture cell units side by side and encoding them en bloc. However, this prior art method utilizes an encoding method which considers statistical properties of an image. So, there is a defect to the effect that encoding efficiencies are quite different in the case of sentences using different characters such as alphanumerical sentences and Chinese character sentences.